In recent years, there have been great advancements in the speed, power, and complexity of integrated circuits. Large scale integrated circuits comprise thousands of devices placed on a single integrated circuit chip. A standard process for electrically isolating the devices on an integrated circuit chip is the local oxidation of silicon process (LOCOS process).
The creation of a prior art LOCOS isolation structure is illustrated in FIG. 1 and in FIG. 2. The structure 100 shown in FIG. 1 comprises a silicon substrate 100. A layer of pad oxide 120 made up of silicon dioxide (SiO2) is placed on the silicon substrate 100. Then a layer 130 of silicon nitride (Si3N4) is placed on the layer pad oxide 120. A mask and etch procedure is used to etch an aperture 140 through the silicon nitride layer 130 and through the pad oxide layer 120 down to the silicon substrate 100. The application of the mask and etch procedure creates the structure 100 shown in FIG. 1.
Then the portions of the silicon substrate 100 that are exposed through aperture 140 are exposed to steam (H2, O2) at a relatively high temperature (e.g., one thousand degrees Celsius (1000° C.)). The oxygen in the steam oxidizes the silicon substrate 100 to form silicon dioxide (SiO2). The oxidation process causes the oxidized portion of the silicon substrate 100 increase in size.
The resulting structure 200 is shown in FIG. 2. The oxidized portion of the silicon substrate 100 is designated with reference numeral 210. As shown in FIG. 2, the oxidized portion 210 of the silicon substrate 100 is sometimes referred to as a field oxide. The field oxide 210 forms an isolation structure or isolation barrier that electrically separates and isolates portions of the integrated circuit chip.
At the edges of the field oxide 210 (near the edges of the silicon nitride portions 130) the thickness of the field oxide tapers off. The maximum thickness of the field oxide 210 (shown by double arrows in FIG. 2) gradually decreases near edges of the field oxide 210 and tapers down to the thickness of the pad oxide 120.
The tapering profile of the edges of the field oxide 210 forms a portion of the field oxide 210 that is known as a “bird's beak.” The bird's beak portion of the field oxide 210 in FIG. 2 is designated with reference numeral 220.
If the bird's peak portion of the field oxide has a relatively graded slow tapering profile the resulting bird's beak profile will have the bird's peak profile 300 shown in FIG. 3. For convenience in description the bird's beak profile 300 will be referred to as a “graded” bird's beak.
If the bird's beak portion of the field oxide has a relatively short quick tapering profile the resulting bird's beak profile will have the bird's beak profile 400 shown in FIG. 4. For convenience in description the bird's beak profile 400 will be referred to as an “abrupt” bird's beak.
In prior art manufacturing processes a thick pad oxide and/or a thin silicon nitride layer will create a graded bird's beak 300. Similarly, in prior art manufacturing processes a thin pad oxide and/or a thick silicon nitride layer will create an abrupt bird's beak 400.
A significant advantage of the abrupt bird's beak is that the abrupt bird's beak takes up less lateral space than a graded bird's beak. This means that there is less space required to form the field oxide isolation structure. Therefore there is more space remaining in the integrated circuit chip for the integrated circuit devices (e.g., transistors). This concept is usually expressed by stating that the abrupt bird's beak provides a better packing density for the integrated circuit devices. A major drawback of the abrupt bird's beak is that the abrupt bird's beak has a lower breakdown voltage.
Conversely, a major advantage of the graded bird's beak is that it provides a higher breakdown voltage. But the graded bird's beak takes up more lateral space than an abrupt bird's beak. This means that the graded bird's beak has a correspondingly worse packing density in the integrated circuit chip for the integrated circuit devices (e.g., transistors).
In prior art manufacturing methods a single value of pad oxide thickness is used and a single value of silicon nitride thickness is used. This means that the designer of an integrated circuit is forced to make a tradeoff between the packing density and the breakdown voltage of the integrated circuit chip. If the packing density is optimized then the breakdown voltage is not optimized. Conversely, if the breakdown voltage is optimized then the packing density is not optimized.
There is a need in the art for an efficient method for manufacturing an integrated circuit that has two different field oxide profiles. In particular, there is a need in the art for a method that is capable of forming both a graded bird's beak profile and an abrupt bird's beak profile with the same field oxidation process.
The present invention provides an efficient method for creating different field oxide profiles in a local oxidation of silicon process (LOCOS process). The method comprises (1) forming a first portion of the field oxide with a first field oxide profile (e.g., an abrupt bird's beak profile) during a field oxide oxidation process, and (2) forming a second portion of the field oxide with a second field oxide profile (e.g., a graded bird's beak profile) during the same field oxide oxidation process.
As will be described more fully below, an advantageous embodiment of the method of the invention uses a first value of a pad oxide thickness and a first value of a silicon nitride thickness to form a first portion of a field oxide having a first field oxide profile. The method simultaneously uses a second value of a pad oxide thickness and a second value of a silicon nitride thickness to form a second portion of the field oxide having a second field oxide profile.
It is an object of the present invention to provide a method for efficiently forming two different field oxide profiles during a field oxide oxidation process.
It is an object of the present invention to provide a manufacturing method for an integrated circuit that gives an integrated circuit designer the flexibility to create an appropriate type of field oxide profile in a desired location.
It is another object of the present invention to provide a manufacturing method for an integrated circuit that creates a graded bird's beak profile in an area where it is more important to have a higher breakdown voltage than a high packing density.
It is another object of the present invention to provide a manufacturing method for an integrated circuit that creates an abrupt bird's beak profile in an area where it is more important to have a higher packing density than a high breakdown voltage.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those persons who are skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Persons who are skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Persons who are skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the Detailed Description of the Invention below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like.
Definitions for certain words and phrases are provided throughout this patent document, those persons of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior uses, as well as future uses, of such defined words and phrases.